The present invention relates generally to natural birth control and, more particularly, to a microprocessor-based instrument for measuring basal (waking) body temperature (BBT) on a daily basis and interpreting the temperature measurement results to indicate the beginning of periods of infertility.
A known method of natural birth control is based on a recognition that basal body temperature in fertile women is variable, and normally follows a cyclical pattern, shifting upwardly near the middle of each menstrual cycle. Within a few days of the occurrance of the upward shift in temperature, sufficient time has elapsed from the occurrance of ovulation, and a period of infertility begins, knowledge of which may be utilized in a program of natural birth control.
Basal body temperature is obtained by taking a temperature measurement at a standard time which ideally is just after waking and before rising. The traditional approach involves taking daily basal body temperature readings with a conventional thermometer, recording the daily measurements in graphical form, and interpreting the graph in order to identify the occurrance of a significant upward shift in temperature.
It will be appreciated that the traditional manual approach is somewhat unreliable for a number of reasons. For one, the accuracy of the temperature reading taken by the woman is sometimes poor. Additionally, it is frequently difficult for the average person to decide when the upward shift in temperature occurs. Another reason for unreliability is that it is inconvenient for most women to take their temperature every day immediately on waking, enter the temperature reading on a graph, and spend some time studying that graph. Taking shortcuts when doing these steps compromises the quality and meaning of the results.
Accordingly, it has previously been proprosed to automate the entire measurement and computation process in a portable device. For example, Letter U.S. Pat. No. 4,151,831 discloses various forms of a microprocessor-based instrument including a probe for making temperature readings, a digital clock for indicating when a temperature reading is to be taken, a memory for storing daily temperature readings, and a programmed microprocessor computing system for interpreting the results and indicating to the user when a period of infertility has begun.
Another example is disclosed in published U.K. patent application No. 80.40786, filed 19 Dec. 1980, and published as No. 2,066,528 on 8 July 1981, claiming the benefit of U.K. patent application No. 79.44063 filed 21 Dec. 1979, naming as inventors Wolff, Abrams, Royston and Everard, and entitled "Measurement of Basal Body Temperature". This British patent specification apparently corresponds to Wolff et al U.S. Pat. No. 4,396,020, issued Aug. 2, 1983. In order to reliably, and on a statistical basis, recognize the upward shift in BBT indicative of the beginning of a period of infertility, the device disclosed in the above-identified Wolff et al U.K. patent application published as 2,066,528 implements a cumulative sum (CUSUM) algorithm described in the literature: J. P. Royston and R. M. Abrams, "An Objective Method for Detecting the Shift in Basal Body Temperature in Women", Biometrics, Vol. 36, No. 2, pp. 217-224 (June 1980).
Underlying the development of the Royston et al algorithm described in the above-identified literature reference, and implemented in the device disclosed in the above-identified U.K. patent application, is the fact that, even with accurate daily temperature measurements, many charts depicting basal body temperature throughout a menstrual cycle do not display a sharp, clear-cut rise. Various different patterns of basal body temperature rise are found, some of which are quite difficult to interpret without benefit of hindsight. The Royston et al article describes a statistical method for detecting an upward shift in basal body temperature, which method is based on the cumulative sum (CUSUM) test previously employed in the context of quality control of production processes.
For its detailed description of the CUSUM test for detecting upward shift in basal body temperature, the above-identified Royston et al article is hereby expressly incorporated by reference. However, in order that the present invention may be better understood, the Royston et al algorithm is now briefly summarized.
The general problem is to begin with a plurality of sample values x.sub.1, x.sub.2 . . . x.sub.r, . . . x.sub.N of a random variable X. In the context of detecting an upward shift in BBT, each of the sample values x.sub.r is simply a daily temperature reading, appropriately corrected for time of day. It is then desired to detect an upward drift of the mean E(X) above some baseline B. As printed out in Royston et al, in general, menstrual cycles in which ovulation has occurred show a biphasic BBT pattern, with a shift from a low post-menstrual level to a higher level around the time of ovulation. Ovulation usually takes place about two weeks before the onset of the next menstrual period. Royston et al refer to temperatures at the lower post-menstrual level to be "pre-ovulatory", and those at the premenstrual higher level as "post-ovulatory", although it is acknowledged that the temperature change does not actually prove that ovulation has occurred.
The baseline temperature is taken as a simple average of a number of daily readings. In the instrument of the present invention, the baseline is preferably taken as a simple average of eight daily readings, commencing on the fourth day following the beginning of a menstrual period. Thus, these eight days may be considered to be a baseline period. Temperature readings for the first three days of a menstrual period are not considered valid because they are sometimes still elevated as a carryover from the previous cycle.
The actual CUSUM test for the purpose of detecting an upward shift begins following the baseline period. The temperature reading for each day is generally compared to the baseline temperature (or, more accurately, to a reference temperature R derived from the baseline temperature). Positive deviations are represented by (x.sub.r -R). The cumulative sum (CUSUM) of positive deviations will eventually become significantly large.
In the details of the implementation, a minimum change term is statistically predetermined, the minimum change term being related to the minimum basal body temperature rise considered to be physiologically significant as indicating a shift truly representative of ovulation. As reported by Royston et al, this minimum BBT rise is approximately 0.2.degree. C. and, for purposes of the CUSUM test, the predetermined minimum change term is 0.1.degree. C.
For purposes of comparison during the CUSUM test, rather than the actual baseline average temperature, a "central reference value" is employed, which is simply the actual baseline average temperature plus the predetermined minimum change term, which is 0.10.degree. C. This "central reference value" corresponds to the "reference temperature" R, introduced above.
The positive deviations (x.sub.r -R) are accumulated day by day, and their cumulative sum (CUSUM) compared to a decision interval, which is also statistically predetermined. In the instrument of the present invention, the decision interval is taken as 0.25.degree. C. On a day when the cumulative sum of positive deviations exceeds the decision interval, the CUSUM test is satisfied, indicating that a period of infertility has commenced, and the user may stop taking daily temperatures until the start of the next menstrual period.
As described in the above-identified Wolff et al U.K. patent application published as No. 2,066,528A, for improved reliability the Royston et al CUSUM test can be enhanced in two particular ways.
First, to reduce the scatter of temperature readings and thus improve the reliability of the CUSUM test, correction should be made to each temperature reading according to the time of measurement. In particular, the rate of rise of post-menstrual temperature of women before waking has been found to be close to 0.1.degree. C. per hour between 6:00 A.M. and 10:00 A.M. Further, it is known that basal body temperature readings are valid only during the approximate period of 6:00 A.M. to 10:00 A.M.
Second, in order to reduce the probability of a false indication of infertility even though the basic CUSUM test is satisfied, it is desirable to require that, not only must the CUSUM test be satisfied, but the deviation of each individual daily temperature reading over the reference value R be positive for at least three consecutive days, including the day on which the CUSUM test is satisfied.
As further described in the above-identified Wolff et al U.K. application, it is desirable to also establish a temperature range within which temperature readings must lie in order to be accepted as valid.
Another example of such an instrument is disclosed in published European patent application No. 0,022,060, published 7 Jan. 1981, based on application No. 80.810166.1 filed 22 May 1980. This European patent application discloses a microprocessor-based system intended for use in natural birth control and including a removable remote temperature probe having its own internal memory and battery.
It will be apparent from the foregoing that several instruments generally similar to the instrument of the present invention have previously been proposed. However, the need for improvement, as a practical matter, remains.
For example, for reasons of minimal circuit complexity (and therefore cost) and flexibility in effecting fine adjustments to the algorithm parameters as additional statistical data become available, it is desirable to employ a programmable microprocessor-based instrument, for example as disclosed in Lester U.S. Pat. No. 4,151,831, rather than a hard-wired instrument as disclosed in the above-identified Wolff et al British application published as No. 2,066,528. Indeed, the specification of the Wolff et al British application recognizes that an instrument might be constructed using a microprocessor chip.
There are, however, a number of specific considerations, particularly as related to considerations of power consumption and user convenience, which are addressed in accordance with the present invention.